1. Field of the Invention
The present invention relates to a digital cross connection apparatus which can be applied to the cross connection of virtual tributary (VT) signals of synchronous optical network (SONET) specifications.
In digital cross connection apparatuses, in particular broad band digital connection apparatuses, it is necessary to be able to cross connect VT signals of various VT sizes corresponding to, for example, the North American 1.5M (Mega bit/second, same below), 3M, and 6M and CEPT (Conference of European Postal and Telecommunications Administration) 2M signals using a common hardware construction. Here, the invention will be explained with reference to primarily VT signals of the SONET specifications.
2. Description of the Related Art
In a digital cross connection apparatus handling SONET signals, use is made of VT signals of a VT format for the frame structure in signal processing in the apparatus. For example, in the case of cross connection of North American 1.5M, 3M, and 6M signals, use is made, for example, of the VT1.5 size as the cross connection unit and it is possible to simultaneously cross connect signals of VT3 and VT6 sizes, as these are whole multiples of the VT1.5 size.
CEPT signals, however, have a VT size of VT2. VT2 is not a whole multiple of VT1.5, so if it is attempted to cross connect VT2 size signals by an apparatus using VT1.5 as the cross connection unit, the efficiency becomes extremely poor. Therefore, cross connection of VT2 had required changing of the switch units and other hardware so as to handle VT2 size cross connection units.
Therefore, in digital cross connection apparatuses handling SONET signals, despite it being considered necessary to cross connect any signal input, whether of the CEPT specification or North American specification, it has not been possible to make common use of the switch units and other hardware for these signals, so these signals have not been handled by the same apparatuses in the past, i.e., separate apparatuses have been necessary.